The idea is known of producing such hollow envelopes from composite material on a basis of fibres impregnated with polymerizable resin. These envelopes which form in particular the skin of glider wings are produced in accordance with a first technique from glass-fibre or other cloth impregnated with resin and after cutting out, draped in hollow moulds, enabling, after polymerization, usually under vacuum, half-envelopes to be obtained. The latter are then assembled by gluing after the parts of the frame of the structure have been put in place between the two half-shells and having been attached to them, for example, by gluing. The main disadvantages of this technique lie in the necessity of a workforce which is large and very well qualified because numerous operations are effected manually. In spite of the high qualifications of the workers, it is found that defects persist of which account must be taken in the design of the part by providing large margins of safety. Finally it is necessary to take the completed part up again in order to carry out finishing operations by hand, e.g. the removal of burrs from the plane of the joint and polishing, these operations leading to an extremely high cost of manufacture the main component of which is the cost of labour but which equally includes a not insignificant factor in the spilling and waste of the raw material employed.
A second method of achieving this type of product has already been proposed, which consists essentially inproducing a winding filament (with threads, sheets or tapes) impregnated with resin over a substantially cylindrical mandrel and then to cut out the blank so obtained along two generatrices of the mandrel in order to obtain sheets which are then arranged in hollow moulds where, flattened against the walls of this mould, they undergo polymerization of their resin. This method enables the labour necessary to the draping of the preceding technique to be avoided, and a certain saving of material, but avoids neither the operations of assembly of the two half-shells obtained nor the finishing operations.
The idea of proceeding by winding filaments over a deformable hollow mandrel has also been proposed. The mandrel is in fact an inflatable envelope over which the winding is effected in the inflated state and then this envelope is deflated whilst still equipped with the unpolymerized winding in order to introduce it into a hollow mould the impression of which corresponds with the shape which is to be obtained. The mandrel is then reinflated in order to flatten the wound skin against the walls of the mould. The winding is then polymerized into shape. This method enables production to be mechanised and hence the cost of it to be reduced but presents an important technical disadvantage in the fact that a sliding of the fibres over one another occurs between the position in which they are wound over the mandrel and the position in which they are put into shape in the mould and fixed by the polymerization of the resin. Whereas the position of winding is known and controlled, as well as the tension of the threads, the final position of the latter as well as their state of strain is not absolutely controlled, which may affect adversely the quality and the anticipated mechanical characteristics of the product.